With Marksmen playable in all other lanes (even jungle!), it's time to open up bot for Mage carries
Call me oldfashioned, but I like some good classic diversity in the team composition. Some big guys who make the frontline, some quick fighters, some magical support, and the guy/gal who shoots. Doesn't have to be arrows.
But when the team weaponry consists solely of a machinegun crossbow (top), double-barrelled shotgun (jungle), roflcopter rockets (mid) and onslaught of bullets (bot), it's becoming less of a fantasy setting and something that begins to resemble Counter-Strike. Four-ADC team would obviously be ridiculous, but competitive LoL has already seen triple-marksman compositions.
The Marksman update was a success. The champions became more diverse and fun to play. Which is important, because unlike every single other class in the game, the number of ADCs in a game cannot go below 1. You can have no tanks, no fighters, no assassins, no mages - you can even use a mage or a tank in place of a true support. But ADCS have been a must-have component of every single team composition for years now.
I'm not saying we have to remove the necessity of having ADCs from the game. It doesn't have to be that big of a change. But now that these mandatory ranged shooters can reliably find their place in other lanes, it's high time we allow other classes to take on the Babysat Carry role.
Mordekaiser was one daring attempt at that, failed at many angles for a variety of reasons. Champion kit changes forcing the duo lane are probably the wrong way to do this. If it's possible by changing any other systems - items, towers, exp share mechanics, gold gain - that should be the way to go.
It doesn't even matter that much which class enters the bot lane. Melee carries, carry mages - anything new would be welcome. Mages are just my suggestion because they seem to be the easiest option.

is the prime example of an AD caster.
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and it's upgrades and
also not AD items.